What causes damage to concrete?

So we've done our evaluation and now we are ready to determine what caused the damage—this is often called troubleshooting. Start by thinking about the basic characteristics of concrete—strong in compression, weak in tension. Therefore, a crack implies that the concrete was in tension. Recognize that the crack is always perpendicular to the direction of the tension—always!

Think of a typical shrinkage crack running diagonally from a re-entrant corner in a concrete slab. The concrete was shrinking back in each direction from the corner and that diagonal crack is perpendicular to the direction of shrinkage. Look at a diagonal crack across the corner of a slab panel where it was run over by heavy traffic or the subbase was poorly compacted—the bending force created tension across the top of the slab. Take a saltine cracker and bend down the corner—it breaks in a diagonal line exactly the same as a concrete slab. I guess you could think of a concrete slab as nothing more than a big cracker!

Here are a few typical concrete problems and their causes:

Corrosion of reinforcing steel: Steel rebar is protected inside concrete because the concrete is very alkaline which prevents rust. But if there are chloride ions present, such as from deicing salts, the chloride destroys the "passivating layer" of alkalinity around the steel, allowing it to rust. Rust has greater volume than steel and the expansion presses against the concrete putting it in tension and causing it to crack and pop off. Chlorides get to the concrete through cracks or by simply penetrating through the concrete's pore structure.

Freeze-thaw disintegration: Concrete is porous, so if water gets in and freezes it breaks off small flakes from the surface. Deicing salts make it worse. This is typically called scaling and it can occur during the first winter and get worse over time. When severe, it can lead to complete destruction of the concrete. Proper air entrainment completely prevents scaling.

Alkali-aggregate reaction—AAR is a result of reactive aggregate in the concrete that forms a gel around the aggregate particle. When that gel gets wet it expands and can destroy the concrete. There are now some lithium products that can mitigate AAR.

Drying shrinkage cracks—Most concrete has more moisture in it when placed than is consumed by the hydration reaction. As that water evaporates, the concrete shrinks—about 0.15 inches over 20 feet, depending on how wet the concrete starts out. If you could hold a slab up in the air while it shrank, it probably wouldn't crack, but since it is on the ground it drags on the subgrade and the shrinkage is restrained and cracks form. Joints in concrete control the drying shrinkage cracks—at least joints let us decide where the crack will form. Learn more about concrete joints.

Plastic shrinkage cracks—When concrete is placed, if the surface is allowed to dry out before the concrete has gained strength, a pattern of cracks will form on the surface. This is also called crazing These cracks are very shallow and narrow and are seldom a serviceability problem—although they can be objectionable to some owners, especially on decorative surfaces. The solution is proper curing, although synthetic fiber reinforcement can reduce this phenomenon.

Blisters/delamination—Bubbled concrete surfaces may be blisters caused by prematurely finishing and sealing the concrete surface and sealing in air and bleed water. This can especially be a problem with air entrained concrete and heavy finishing equipment.

Cracks from structural loads-Here's how reinforced concrete works: the steel reinforcement is completely useless until the concrete cracks. That crack my be very narrow, even invisible, but until the concrete cracks the steel doesn't start getting pulled to hold the concrete together. If there is no steel, inadequate steel, or the loads are too heavy (either during construction or in service), then the cracks can become wider. For a slab on ground that may not be significant, but for a wall or beam or column, cracks may signal structural problems.